feat: Add Bitcoin wallet provider support

[nud3l]

Summary

Agentkit currently supports EVM and Solana wallet providers but has no Bitcoin wallet provider. Adding a BtcWalletProvider would allow action providers to interact with the Bitcoin network directly, rather than requiring users to supply BTC addresses as input parameters.

Motivation

• The BOB Gateway action provider (feat: add BOB Gateway BTC offramp action provider #983) currently requires the agent/user to provide a BTC address manually for EVM-to-BTC swaps. A BTC wallet provider would let the agent manage its own BTC address.
• A BTC wallet provider would also unlock BTC-to-EVM onramping via BOB Gateway (and potentially other bridge providers).
• Bitcoin is the largest crypto network by market cap and a common target for cross-chain operations.

Proposed scope

A BtcWalletProvider extending WalletProvider with:

• getAddress() returning a BTC address (P2WPKH/P2TR)
• getNetwork() returning a Network with protocolFamily: "btc" and appropriate network IDs (e.g. bitcoin-mainnet, bitcoin-testnet)
• getBalance() fetching the BTC balance via an indexer API
• nativeTransfer() signing and broadcasting a BTC transaction
• Bitcoin network definitions in src/network/ (similar to src/network/svm.ts for Solana)

The initial implementation would follow the local keypair pattern (like SolanaKeypairWalletProvider), since the CDP SDK currently only supports EVM and Solana, and Privy's BTC support (bitcoin-segwit) is only available via their REST API, not yet in the server-auth SDK types.

As CDP and Privy add Bitcoin wallet support to their SDKs, managed key variants (CdpBtcWalletProvider, PrivyBtcWalletProvider) could be added to match the existing pattern for EVM and Solana.

Contribution

We (BOB) would be happy to contribute this implementation. Is this something the agentkit team would be interested in?

Are there preferences on the technical side for a Bitcoin signing library (e.g. bitcoinjs-lib, @scure/btc-signer) or indexer/broadcast API (e.g. Blockstream/Esplora, mempool.space)?